Unicode::UCD::prop_invmap(): Return deltas for the 'dm' property

Unicode::UCD::prop_invmap(): Return deltas for the 'dm' property

Earlier commits caused the return of prop_invmap() for certain
properties to return deltas from code points instead of the code points
themselves, for compactness of storage and speed of searching.  This
causes the same for the 'dm' property, for consistency with the others,
even though the space savings is not large for this one; essentially the
same code can be used for the two types now; instead of an application
having to have special cases.